1. Field of the Invention
The present invention relates to a process for removing inorganic compounds from polyester glycol recovery bottoms formed as a by-product in the manufacture of poly(ethylene terephthalate) using a centrifuge to separate the inorganic compounds from the bottoms. This process improves not only the removal of inorganic compounds, but also provides for insolubilizing inorganics, such as antimony, and then centrifugally removing the resulting antimony precipitate. In particular, the process includes forming the glycol recovery bottoms by distilling spent glycol produced as a by-product of polyester to have a solids concentration from about 15% to about 45%, and centrifuging the glycol recovery bottoms to remove the inorganic compounds. In addition to the centrifugal removal of inorganic compounds, the addition of a precipitating reagent to the glycol recovery bottoms before the centrifuging of the glycol recovery bottoms insolubilizes other inorganic compounds which can also be centrifugally removed. In particular, it has been found that adding a mole ratio of about 2 P/Sb as phosphoric acid before centrifuging permits the removal of antimony and increases the amount of removal of TiO.sub.2.
2. Prior Art
Poly(ethylene terephthalate) (PET) is a commercial film and fiber forming polyester which is generally manufactured by reacting a lower dialkyl ester of a dicarboxylic acid such as dimethyl terephthalate (DMT) with a molar excess of ethylene glycol (EG) in the presence of a catalyst (such as compounds of manganese, zinc, calcium, magnesium, for example). A delustrant such as TiO.sub.2 is also added. These components undergo ester interchange to yield bishydroxyethyl terephthalate (BHET) and methanol. Phosphorus is preferably added to sequester the ester interchange catalyst. Alternatively, BHET may be produced by the direct esterification of terephthalic acid (TA) also with an excess of ethylene glycol (EG). The BHET is then polymerized by a polycondensation reaction in the presence of a suitable polymerization catalyst (such as an antimony compound). From this reaction PET and spent ethylene glycol are formed. Because the polycondensation reaction is reversible, the excess spent glycol is removed as it is evolved, thus forcing the reaction toward the formation of the polyester. Large quantities of spent ethylene glycol distillate, hereinafter "spent glycol", are produced during the polymerization reaction.
Unfortunately, the spent glycol contains impurities which make it unsuitable, as is, for the manufacture of PET or for other uses such as the production of polyols, polyurethane foams, and unsaturated polyester resins, etc. For instance, a typical spent glycol contains mostly ethylene glycol, along with various quantities of diethylene glycol, water, polyester oligomers, and inorganic compounds. Contained in the inorganic organic compounds are titanium dioxide, phosphorus and about 50 to 500 ppm of antimony. Accordingly, it is standard practice in the industry to purify the spent glycol, recovering distilled glycol and reusing the recovered glycol in the manufacture of PET. A by-product of this purification is a mass known in the industry as polyester glycol recovery bottoms, in which is contained inorganic compounds. The inorganics are concentrated 30 to 50 times in the bottoms by the ethylene glycol recovery process. The presence of some inorganic compounds such as antimony compounds or titanium dioxide makes the by-product unusable. Removal of these inorganic compounds has proved troublesome to date.
In prior art processes, the spent glycol is passed through a light-ends removal column (water stripper column) under reduced pressure and at a column top temperature of about 75.degree. C. The glycol is then fractionally distilled in one or more columns or in flash or thin-film evaporators under vacuum condition, ranging from about 20 to 200 mm mercury absolute pressure and at a temperature of from about 130.degree.-160.degree. C. The glycols including ethylene and diethylene glycol are recovered from the spent glycol as a overhead stream. The glycols are further distilled to separate the ethylene glycol from the diethylene glycol. In a typical distillation about 90 to 95% of the ethylene glycol present in the spent glycol is recovered in the overhead stream, the balance remaining in the bottoms and dehydrated to diethylene glycol.
As the glycols are fractionally distilled, most of the residual impurities remain in the bottom of the column or the evaporator taking the form of a waxy granulated mass and will be referenced to as "glycol recovery bottoms or bottoms". A typical bottom includes about 98% terephthalate esters and glycols (ethylene and diethylene) and about 2% of inorganic compounds of Sb and P including TiO.sub.2.
The terephthalate ester and diethylene glycol of the polyester bottoms are known to have commercial applications in markets including polyol/polyurethane and unsaturated polyester resin which use crude terephthalate streams as raw materials. However, specifically the inorganic compounds of Sb and TiO.sub.2, have been found intolerable in such applications. No known problem has ever been associated to date with phosphorus in downstream end uses. Without such commercial applications, polyester glycol recovery bottoms containing these inorganic compounds require disposal by burying or incinerating. However, antimony is known to be highly toxic. The Environmental Protection Agency (EPA) has taken the position that the presence of antimony constitutes an environmental and subsequent health hazard. A maximum acceptable limit of 5 PPM of antimony has been established by the EPA for effluent discharged into a waterway. A problem with incinerating polyester bottoms is the high volatility of antimony oxides. Pretreating for burying or incinerating to mitigate antimony-related problems becomes enormously expensive.
The following references are directed to various methods for purifying spent glycol and methods of separating the inorganic impurities from the glycol recovery bottoms.
U.S. Pat. No. 2,788,373 to Mills, Jr. et al discloses a process for recovering ethylene glycol from spent glycol using an acid to precipitate out soluble solid matter. In particular, the spent glycol is first diluted with water and a mineral acid such as phosphoric acid is added to precipitate out soluble solid matter usually terephthalate salts. The precipitated solid matter is removed by decantation or filtration.
U.S. Pat. No. 4,013,519 to Hoppert et al discloses recovering antimony from the polyester bottoms by alkaline hydrolysis, acidification and filtration to remove antimony sulfide precipitate from spent glycol. Such a process is disclosed to remove terephthalic acid from the polyester bottoms without contamination by the antimony. The uncontaminated terephthalic acid can be recycled for polyester manufacture or other use.
U.S. Pat. No. 4,046,688 to Cunningham et al discloses a process to recover dissolved antimony using a strong acid and/or a strong base in an ion exchange resin to absorb the antimony. This process is done after the spent glycol has been filtered to remove suspended solids.
U.S. Pat. No. 4,100,253 to Dougherty et al discloses a process to recover antimony from spent glycol by incineration. In particular, the polyester bottoms are incinerated to produce an ash and then the ash is contacted with water to form an ash-water from which the antimony compounds can be recovered.
U.S. Pat. No. 4,118,582 to Walker discloses a process to recover antimony from spent glycol by precipitating out the antimony with an alkali metal borohydride. The process can be enhanced by adding a strong inorganic base.
Improvements which remove just antimony but not titanium dioxide, for example, partially solves the foregoing problem. However, one goal of the present invention is to remove both the antimony and titanium dioxide.
There remains a need to develop a process for the purification of polyester glycol recovery bottoms which minimizes the problems of disposal of the polyester bottoms so the polyester bottoms can be recycled for other uses. Especially desired is an improved process wherein contaminates present in glycol recovery bottoms, such as antimony and titanium dioxide can be economically removed.